Among the numerous challenges facing the environmental cleanup and detoxification industry is the problem of removing contaminants from soil. In particular, there exists a substantial need for improvement in the methods for removal of volatile hydrocarbons (VOC's) from contaminated soils. Because of the large number of underground storage tanks, many having leaks, the need for an effective means of soil decontamination is significant.
Among the techniques that are presently employed to remove contaminants such as VOC's from soil is a process known as extraction or soil venting. This process involves pulling air through the soil with blowers, thereby causing soil-borne VOC's to enter the airstream and be pulled to the surface and out of the soil. Once the airstream containing the VOC's is obtained, removal or decomposition of the VOC's is necessary.
Unfortunately, most pollution cleanup systems currently in operation simply change the phase of the contaminant. For example, in many older systems, VOC's which are stripped from ground water or soils are simply discharged into the atmosphere without treatment. This contributes to smog formation and low altitude ozone formation. In recent years, it has become widely recognized that it is unacceptable to simply remove the contaminant from soil or water and release it into the atmosphere. Rather, it has become recognized that there is a need to provide further processing to break down the contaminant to thereby minimize its effects on the environment.
To that end, numerous technologies exist that are employed to control VOC emissions in soil venting and similar contaminant removal systems. These include activated carbon filtration, high temperature thermal incineration and catalytic destruction. Although each of the systems is known to be efficient if the operating parameters are maintained within an optimum range, they all suffer from serious disadvantages.
For example, although activated carbon can remove VOC's from airstreams at very high efficiencies, carbon has only a limited capacity to adsorb any particular compound and quickly becomes saturated. Once the saturation limit is reached, the carbon bed will "break-through" and cease to be functional as a treatment system. When break-through occurs, the carbon bed must be regenerated or disposed of. Contaminated carbon is considered a hazardous waste under federal statutes and thus becomes very expensive to dispose of. Although some carbon companies offer regeneration facilities or on-site steam regeneration, the VOC's captured by the carbon still remain. Thus, when a VOC-saturated carbon bed is regenerated, condensed steam containing the VOC's removed from the carbon becomes hazardous waste. Furthermore, since the VOC's remain unchanged chemically, having been carried from soil ultimately into steam, only the phase of the contaminant has been changed. As an additional drawback, large volumes of carbon are needed for vapor phase systems, particularly if the VOC concentrations at the contaminated site exceed a few parts per million (ppm).
Alternatively, many contaminants can be incinerated at high temperatures. At temperatures exceeding about 1400.degree. F., the molecular bonds which hold organic molecules together fail. This principle can be applied in thermal incineration systems in which the process airstream containing VOC's is heated to above the decomposition temperature and maintained there for a certain residence time. This allows all of the organic contaminants contained in the process airstream to be destroyed. Thermal incineration is typically carried out using fluidized bed combustors, rotary kilns and special furnaces. Unfortunately, effective decomposition of contaminants present at the concentrations found in typical pollution control systems requires that large amounts of fuel be added to the contaminated airstream in order to achieve the necessary temperatures. Thermal incineration systems are also very expensive to maintain and operate and suffer from operation expenses which vary drastically with the fluctuating price of fuel supplies.
In an alternative incineration technique, catalytic reactions have been used. Catalytic incineration works on the same principle as thermal technologies, except that a catalyst is employed to lower the temperature at which the organic contaminants are destroyed. Many catalytic incinerators include heat exchangers to improve the process efficiency. Unfortunately, catalytic incinerators still require large amounts of fuel or electricity when VOC concentrations in the process airstream are low. Such systems also tend to be very expensive, complicated and subject to catalyst poisoning, a condition that occurs when the catalyst is transformed by a chemical reaction rather than facilitating it. In systems using platinum catalyst, for example, lead contaminants act as a poison. Once poisoned, the catalysts must be replaced, adding further significant expenses to the operation of such incinerators.
As an alternative to soil venting in which contaminates are removed from soil and then subsequently processed, a variety of in situ methods for the biological digestion of various contaminants have been used. These systems are often referred to as bioremediation systems. For example, U.S. Pat. No. 4,850,745 to Hater et al. describes a system for treating soil contaminated with petroleum hydrocarbons. The system comprises an excavated cavity containing a layer of gravel which covers a bacterial culture capable of degrading petroleum hydrocarbons. A piping system capable of distributing nutrients directly to the cultures and a means for providing air flow through the area containing the cultures are provided as well. In use, air from the surface is pulled through the contaminated soil and directly into the bacterial cultures, thereby entraining contaminants and allowing them to be digested by the bacteria prior to release into the atmosphere. Although such a system may be useful for cleaning contaminated soil in a limited area, i.e., such as that lying directly beneath a leaking petroleum storage tank, the system suffers from the disadvantage of requiring excavation and installation prior to the installation of the storage tank, the inability to easily alter or expand the treatment area, and the inability to provide adequate process controls to maintain the decontamination process within optimum reaction parameters.
Thus, a need exists for a treatment system having a low capital cost, a high removal efficiency, low maintenance, and minimal energy requirements. In addition, a need exists for a relatively simple yet effective means for removing contaminants, such as VOC's from a process airstream.